Transition Elements

Transition elements are the d-block metals occupying Groups 3-12 of the periodic table. The PMDC MDCAT 2026 syllabus expects you to know their electronic configuration quirks, why they show variable oxidation states, why their compounds are coloured, and their characteristic catalytic and complex-forming behaviour. This chapter is small but high-yield — expect 1-2 MCQs per paper.

PMC Table of Specifications. This chapter has one PMDC subtopic — Electronic Structure of d-block Elements. We cover it in depth below: configuration, variable oxidation states, colour, complex-ion formation, magnetic behaviour, and catalytic activity.

Electronic Structure of d-block Elements

A transition element is defined as one whose atom or one of its common ions has a partially filled d-subshell. By this strict IUPAC definition, Zn (Group 12) is sometimes excluded because its 3d shell is full in both the atom and Zn²⁺. For MDCAT purposes, however, the entire d-block from Sc to Zn is studied as transition elements.

The 3d series (first transition series)

The first transition series spans atomic numbers 21-30 (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn). Their general electronic configuration is [Ar] 3d^1-10 4s^1-2. The 4s orbital fills before 3d (Aufbau), but 4s is the higher-energy orbital once 3d is occupied — so when these elements ionise, electrons are lost from 4s first, not 3d.

Configuration cheat sheet (3d series)

Sc: [Ar] 3d¹ 4s²
Ti: [Ar] 3d² 4s²
V: [Ar] 3d³ 4s²
Cr: [Ar] 3d⁵ 4s¹ — not 3d⁴4s²; half-filled 3d is extra stable
Mn: [Ar] 3d⁵ 4s²
Fe: [Ar] 3d⁶ 4s²
Co: [Ar] 3d⁷ 4s²
Ni: [Ar] 3d⁸ 4s²
Cu: [Ar] 3d¹⁰ 4s¹ — fully-filled 3d is extra stable
Zn: [Ar] 3d¹⁰ 4s²

Common trap. When forming Fe²⁺, electrons are removed from 4s first, giving [Ar] 3d⁶. Fe³⁺ = [Ar] 3d⁵ (half-filled, extra stable — this is why Fe³⁺ is the more stable iron ion in aqueous oxidising conditions). Many candidates wrongly write 4s²3d⁴ for Fe²⁺.

Variable oxidation states

Transition metals show several oxidation states because the energies of 4s and 3d electrons are very close — both are available for bonding. The maximum oxidation state generally rises across the series until manganese (+7), then falls.

Sc: +3 only
Ti: +2, +3, +4
V: +2, +3, +4, +5
Cr: +2, +3, +6 (oxidising as in CrO₄²⁻, Cr₂O₇²⁻)
Mn: +2, +3, +4, +6, +7 (in KMnO₄, very strong oxidiser)
Fe: +2 (ferrous, pale-green Fe²⁺), +3 (ferric, yellow-brown Fe³⁺)
Co: +2, +3
Ni: +2 mainly
Cu: +1, +2 (Cu⁺ is colourless because 3d¹⁰; Cu²⁺ is blue)
Zn: +2 only (3d full in both atom and ion — not a "true" transition element)

Why their compounds are coloured

When a transition-metal ion sits in a ligand field (water, NH₃, Cl⁻ etc.), its d-orbitals split into two sets of slightly different energies. An electron can absorb a photon of visible light to jump from the lower set to the upper set — a d-d transition. The complementary colour of the absorbed wavelength is what we see.

Common ion colours

Ti³⁺ (3d¹) — purple
V³⁺ (3d²) — green; VO₂⁺ — yellow
Cr³⁺ (3d³) — green; Cr₂O₇²⁻ — orange; CrO₄²⁻ — yellow
Mn²⁺ (3d⁵) — very pale pink; MnO₄⁻ — deep purple
Fe²⁺ (3d⁶) — pale green; Fe³⁺ (3d⁵) — yellow-brown
Co²⁺ (3d⁷) — pink (octahedral aqua) / blue (tetrahedral chloro)
Ni²⁺ (3d⁸) — green
Cu²⁺ (3d⁹) — blue; Cu⁺ — colourless
Sc³⁺, Ti⁴⁺, Zn²⁺ — colourless (no partially filled d, no d-d transition)

Complex-ion (coordination compound) formation

Transition-metal cations are small, highly charged, and have empty d-orbitals — ideal Lewis acids that accept electron pairs from ligands. The result is a complex ion like [Cu(NH₃)₄]²⁺ or [Fe(CN)₆]³⁻.

Ligand: A neutral molecule or ion with at least one lone pair that bonds to the central metal. Examples: H₂O, NH₃, Cl⁻, CN⁻, OH⁻, en (ethylenediamine), EDTA⁴⁻.
Coordination number: The number of donor atoms bonded to the central metal. Common values are 4 (tetrahedral / square planar) and 6 (octahedral, most common for the 3d series).
Mono-, bi-, polydentate: Number of donor atoms a single ligand provides. Cl⁻ is monodentate; ethylenediamine is bidentate (two N donors); EDTA⁴⁻ is hexadentate.

Magnetic properties

Transition-metal ions with one or more unpaired d-electrons are paramagnetic — they are weakly attracted to a magnetic field. Ions with no unpaired electrons (Sc³⁺, Zn²⁺, Cu⁺) are diamagnetic. The magnetic moment for spin-only systems is μ = √[n(n+2)] Bohr magnetons, where n is the number of unpaired electrons.

Catalytic activity

Transition metals are excellent industrial catalysts because they readily switch between oxidation states (lending and accepting electrons) and offer surface sites for reactant adsorption.

Iron — Haber process for NH₃ synthesis (N₂ + 3H₂ ⇌ 2NH₃).
Vanadium(V) oxide V₂O₅ — Contact process for SO₃ (and hence H₂SO₄).
Nickel — hydrogenation of vegetable oils to make margarine.
Platinum-rhodium — Ostwald process (NH₃ → NO → HNO₃) and catalytic converters.
Manganese(IV) oxide MnO₂ — decomposition of H₂O₂ and KClO₃.

Memory aid for "why coloured?". "Empty d sees no light." If the d-subshell is empty (Sc³⁺, Ti⁴⁺) or completely full (Zn²⁺, Cu⁺), no d-d transition is possible — the ion is colourless. Only partially-filled d gives colour.

General properties of transition metals

All are metallic: hard, dense, high melting and boiling points (loss of d-electrons to the metallic bond).
Generally good conductors of heat and electricity.
Many show multiple oxidation states (variable valency).
Most form coloured ions/compounds.
Most are paramagnetic; a few (Fe, Co, Ni) are ferromagnetic.
Form complex ions with Lewis-base ligands.
Many act as industrial catalysts.
Form alloys readily (steel, brass, bronze).

Exam-favourite trap. "Why is Zn not a typical transition element?" The PMDC-friendly answer: its 3d subshell is completely filled (3d¹⁰) in both the neutral atom and its only common ion Zn²⁺. Hence no d-d transitions, colourless compounds, only one oxidation state, and few of the typical transition properties.

Worked MCQs

Five MCQs covering the high-yield testing patterns for transition elements.

Q1. The ground-state electronic configuration of chromium (Z = 24) is:

[Ar] 3d⁴ 4s²
[Ar] 3d⁵ 4s¹
[Ar] 3d⁶
[Ar] 3d³ 4s² 4p¹

A half-filled 3d subshell is extra stable, so chromium promotes one 4s electron to give [Ar] 3d⁵ 4s¹ rather than the Aufbau-predicted 3d⁴ 4s². Copper (Z = 29) shows the same anomaly with [Ar] 3d¹⁰ 4s¹.

Q2. Which of the following ions is colourless in aqueous solution?

Ti³⁺
Cu²⁺
Zn²⁺
Fe³⁺

Zn²⁺ has a fully-filled 3d¹⁰ configuration — no partially filled d-subshell, so no d-d electronic transitions, so no absorption in the visible region, so colourless. Sc³⁺, Ti⁴⁺ and Cu⁺ are colourless for the same reason (empty or full d).

Q3. The catalyst used in the Haber process for ammonia synthesis is:

V₂O₅
Pt-Rh gauze
Finely-divided iron with K₂O / Al₂O₃ promoters
Ni

The Haber process uses iron with promoters at ~450 °C and 200 atm. V₂O₅ is the Contact-process catalyst for SO₂ → SO₃; Pt-Rh is for the Ostwald process; Ni is used in vegetable-oil hydrogenation.

Q4. The number of unpaired electrons in Fe³⁺ is:

Fe is [Ar] 3d⁶ 4s². Fe³⁺ loses 4s² and one 3d electron, giving [Ar] 3d⁵ — five unpaired electrons (Hund's rule), which is why Fe³⁺ is strongly paramagnetic and the half-filled state contributes to its stability.

Q5. In the complex ion [Cu(NH₃)₄]²⁺, the coordination number of copper is:

Coordination number is the number of donor atoms bonded to the central metal. Four NH₃ ligands each donate one lone pair through nitrogen, so the coordination number is 4. The geometry is square planar (typical for d⁹ Cu²⁺).

Quick Recap

Transition elements = d-block, partially filled d-subshell in atom or common ion.
Cr and Cu have anomalous configurations (3d⁵4s¹ and 3d¹⁰4s¹) due to half- and fully-filled stability.
Ionisation removes 4s electrons before 3d.
Variable oxidation states (Mn up to +7) because 4s and 3d are close in energy.
Colour comes from d-d transitions in a ligand field; empty or full d → colourless.
Paramagnetism scales with the number of unpaired d-electrons.
Complex ions form because of small, highly-charged cations with empty d-orbitals (Lewis acids).
Common catalysts: Fe (Haber), V₂O₅ (Contact), Ni (hydrogenation), Pt-Rh (Ostwald, catalytic converter), MnO₂ (H₂O₂ / KClO₃).

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